Abstract : The tumor microenvironment contains immune cells that can recognize and destroy tumors. However, the tumor not only manages to evade the immune system through various mechanisms, but also creates a microenvironment favorable to tumor progression by altering the function of infiltrating immune cells. Macrophages, which are immune cells of myeloid origin, are major players of tumor immunity. Macrophages are divided into two subgroups as M1 (pro-inflammatory) and M2 (anti-inflammatory) macrophages. In general, M1 macrophages are potent tumor-fighting cells, whereas M2 macrophages are tumor-promoting cells. The tumor environment is usually dominated by tumor-promoting M2 macrophages. The mechanisms that govern the switch betweenM1 and M2 macrophages are poorly understood. We have recently made the exciting discovery that macrophages that lack the cholesterol transporter ABCG1 become potent tumor-fighting M1 macrophages and inhibit the progression of bladder cancer in mice. This important finding demonstrates that ABCG1 can modulate macrophage polarization and suggests that ABCG1 in macrophages could be a potential new target for cancer immunotherapy.